Two major requirements for reinforcing metallic cords used in vehicle tires are a high degree of penetration of rubber and high compressive rigidity, the latter having an influence on the cornering performance of the vehicle. But in today's such reinforcing metallic cords, the above two requirements tend to conflict each other. For example, while a steel cord disclosed in Patent publication 1, of which the element wires have corrugations formed with a pitch smaller than their twist pitch, are high in the degree of penetration of rubber, such a cord is lower in compressive rigidity than a metallic cord of which the element wires have no such corrugations (which is called a compact cord). This is because even if the corrugations are formed with a small pitch and/or are two-dimensional corrugations, when element wires are twisted into the cord, three-dimensional large waves having a pith equal to the twist pitch are formed, so that the bellies of the bent portions of the wires face radially outwardly at intervals equal to the twist pitch. These portions are less resistant to compressive loads, thus lowering the compressive rigidity of the cord. If the corrugations of the element wires are large in height before the wires are twisted together, the compressive rigidity of the cord tends to be low over its entire length, irrespective of whether the bellies of the element wires are facing radially inwardly or outwardly.
Patent publications 3 to 5 teach the concept of improving the degree of penetration of rubber and thus its corrosion resistance without corrugating element wires before twisting the wires together. But for this purpose, it is necessary to twist only one of the strands of element wires before twisting the strands together. This increases the facility cost and manufacturing cost. Since the cord of Patent document 2 is a cord with an open construction, it shows high elongation under low loads. This may cause adjacent cords to contact each other when they are set in rubber to reinforce the rubber, because they have a tendency to curl and are unstable.
In order to increase the twist rate, the steel cord of Patent document 1 comprises a first sub-strand of two-parallel element wires each formed with fine two- or three-dimensional corrugations and bound together so as to extend substantially parallel to each other, and a second strand formed by twisting two element wires together, wherein the first sub-strand and the second strand are twisted together with a pitch greater than the pitch with which the element wires of the first sub-strand are corrugated.
The steel cord of Patent document 2 comprises two strands each comprising two or three element wires or sub-strands that are arranged in parallel to each other while being in contact with each other, and twisted together so as to be substantially parallel to each other while maintaining gaps of 8 to 30% of the diameter of the element wires or sub-strands therebetween.
The steel cord of Patent document 3 is formed by twisting a strand of two linear members and two parallel linear members together to improve the degree of penetration of rubber into the cord, while maintaining high elasticity.
The steel cord of Patent document 4 comprises a first group of two element wires and a second group of three element wires, all extending parallel to the axis of the cord. Any two cross-sections of the cord separated longitudinally by a distance of a half pitch are substantially in the shape of an umbrella and an inverted umbrella.
The steel cord of Patent document 5 is formed by helically wrapping a second strand of three wires around a first strand of two wires having a twist pitch of 50 mm such that the second strand has a smaller twist pitch than the first strand, and a gap of 10% or over is formed between the first and second strands in every twist pitch of the first strand.
[Patent document 1] JP patent publication 06-73672
[Patent document 2] JP patent publication 60-10151
[Patent document 3] JP patent publication 61-01554
[Patent document 4] JP patent publication 03-23673
[Patent document 5] JP patent 3174803